Inspired by a discussion with Christoph Hellwig, I tried to recreate a patch that he did a few years ago to add support for writing to a mounted cramfs file system. It still has known problems (and likely unknown ones), but should be good enough for practical use. I've been able to boot a full Ubuntu installation from a cramfs image and work with it normally. The intention is to use it for instance on read-only root file systems like CD-ROM, or on compressed initrd images. In either case, no data is written back to the medium, but remains in the page/inode/dentry cache, like ramfs does. Many existing systems currently use unionfs or aufs for this purpose, by overlaying a tmpfs over a read-only file system like cramfs, squashfs or iso9660. IMHO, it would be a much nicer solution to not require unionfs for a simple case like this, but rather have support for it in the file system. If people find this useful, we can do the same in other read-only file system. Writing to existing files is broken in at least two corner cases, and I'm still looking for a solution here: When you truncate an on-disk to make it larger, reading beyond the old end of the file will make cramfs try to read from disk instead of filling with zeroes. I'm not sure if this can be solved without adding additional members to the inode structure (using a private inode cache) to remember the end of the on-disk file. Deleting a preexisting file currently does not free the inode and page cache for that file, which I assume is easy to fix. Also, the i_nlink field of directories is always 1, and has always been on cramfs. Getting the count right should simplify the code a bit and make it more correct according to posix, but will cost a bit of performance on 'stat'. The patch series also lives on git://git.kernel.org:/pub/scm/linux/kernel/git/arnd/playground.git cramfs Comments? Arnd <>< -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
